Since thin-film fabrication techniques were first used to produce MEMS devices, one of the areas that has seen significant research is the reduction of film stresses and stress gradients. A result of thin-film processes is that out-of-plane deformation of freestanding micro machined films could negatively impact the performance of MEMS devices. The principal source of contour errors in micro machined structures is residual strain that results from thin-film fabrication and structural release. Surface micro machined films are deposited at temperatures that are sometimes significantly above ambient, which create the stresses due to the difference in the coefficient of thermal expansion between different material types when the substrate is cooled.
This residual stress for the material that is added to the substrate is usually tensile. Simply supported structures results in an Euler's type of eccentric loading, which causes suspended beams to deflect downward into the area where the sacrificial material had been removed. One example of this is illustrated in FIGS. 1A-1B where a beam 12 is deposited on a sacrificial material 10 and a substrate 14. When the sacrificial material 10 has been removed from below the beam 12, the beam 12 is supported at each end by a substrate 14 and is deflected downward into the trench 16.
Another problem that occurs from residual stresses in beam 12 occurs when releasing the beam 12. If the stresses are high enough, beam 12 collapses downward and impedes the removal process of the sacrificial material 10. This increases the amount of time that is required in the etching process. The increase in processing time impacts the process throughput and slows down production. It also requires a higher selectivity for the enchants in order for the sacrificial material 10 to be removed without etching into the rest of the device.
Existing technologies used to reduce the effects of residual stresses have been in the areas of ion bombardment to soften the material as disclosed in R. Nowack et al, “Post—deposition reduction of internal stress in thin films: the case of HfN coatings bombarded with Au ions”, Mater. Lett. Vol, 33, no. 1-2, pp 31-36, 1997, which is herein incorporated by reference in its entirety, and device design to account for the deformations that occur due to the residual stresses as disclosed in F. Yuan et al, “Using thin films to produce precision, figured x-ray optics”, Thin Solid Films, vol. 220, no. 1-2, pp 284-288, 1992, which is herein incorporated by reference in its entirety. Annealing the materials after processing is another method that has been commonly used to reduce residual stresses.
Unfortunately, there are problems with these existing technologies. For example, ion bombardment can damage the dielectric properties of the material and can alter the behavioral characteristics of the material. Annealing may relieve the stress of the layer, but may cause other stress in other layers through the heating and cooling. The annealing can change the properties of the material from amorphous to crystalline. If dopants are present in the material, the annealing may cause additional driving of the dopants within the material which may not be desirable.